This study reports on our steps toward achieving a conceptually coherent and empirically validated learning progression for carbon cycling in socio-ecological systems. It describes an iterative process of designing and analyzing assessment and interview data from students in upper elementary through high school. The product of our development process--the learning progression itself--is a story about how learners from upper elementary grades through high school develop understanding in an important and complex domain: biogeochemical processes that transform carbon in socio-ecological systems at multiple scales. These processes: (a) generate organic carbon (photosynthesis), (b) transform organic carbon (biosynthesis, digestion, food webs, carbon sequestration), and (c) oxidize organic carbon (cellular respiration, combustion). The primary cause of global climate change is the current worldwide imbalance among these processes. We identified Levels of Achievement, which described patterns in the way students made progress toward more sophisticated reasoning about these processes. Younger learners perceived a world where events occurred at a macroscopic scale and carbon sources, such as foods and fuels, were treated as enablers of life processes and combustion rather than sources of matter transformed by those processes. Students at the transitional levels--levels 2 and 3--traced matter in terms of materials changed by hidden mechanisms (level 2) or changed by chemical processes (level 3). More advanced students (level 4) used chemical models to trace matter through hierarchically organized systems that connected organisms and inanimate matter. Although level 4 reasoning is consistent with current national standards, few high school students reasoned this way consistently. We discuss further plans for conceptual and empirical validation of the learning progression. (Contains 6 tables, 3 figures and 4 notes.)